Research Highlights

Unlocking Fire Ice

PNNL-developed software used to study a new way of producing methane hydrates—a massive and largely untapped source of energy

PNNL is using STOMP to model a recent DOE experiment in Alaska to improve our understanding of how the guest-molecule exchange process works to produce methane hydrate gas.

Locked away in ice cages on the ocean floor and under Arctic permafrost sits a vast amount of energy: methane hydrates. Dubbed “fire ice” because you can burn the methane while it’s in the ice, methane hydrates hold a massive supply of natural gas. Now, using a new version of PNNL’s Subsurface Transport Over Multiple Phases (STOMP) simulator, researchers are studying a guest-molecule exchange process conceived at PNNL that could unlock ice cages and help open up this immense source of potential energy.

Why it matters

Methane hydrates hold massive energy content—possibly more than all other fossil fuels combined.1 Despite this, commercial production is practically non-existent because of questions about how to produce methane hydrates cost-effectively and in an environmentally friendly manner. This research could provide an environmentally sound alternative to unlocking the ice cages. And while there’s no shortage of natural gas right now, that actually makes this the perfect time for the research, according to PNNL's Mark White, principal investigator.

"We have a window of time to investigate numerically and through experimentation these new resources that are not tapped yet," White said. "It’s not a needed resource today, but it will be in the future, and we have this opportunity to get it right before it’s needed."

Methods

The guest-molecule exchange process works by injecting a mixture of carbon dioxide and nitrogen into a natural gas hydrate bearing formation, releasing the methane and replacing it with either carbon dioxide or nitrogen, which maintains the hydrate’s ice “cage” structure and protects the formation.

“You’re replacing something that’s valuable to you—the methane—with something that’s not—the carbon dioxide and nitrogen,” says White. “It’s important to maintain the cages because that prevents the hydrate-bearing formation from collapsing, which would be an environmental disaster. That’s what they want to avoid, and that’s why this technique is so attractive”.

While the process sounds straightforward, it’s only been done in experiments, there’s very little data, and many questions remain: What actually takes place inside the well after injection? How long do you pump? How much do you pump? What’s the right mixture of carbon dioxide and nitrogen? Too much carbon dioxide can plug the well; too much nitrogen can threaten substrate stability. That’s where STOMP comes in. By accurately modeling the processes in the well, STOMP can make predictions about behavior that will answer these and other key questions holding the technology back.

The ultimate goal of this research, which is being funded by the DOE Office of Fossil Energy Research and Development, Korea Institute of Geoscience and Mineral Resources, and ConocoPhillips is to improve understanding of the guest-molecule exchange process and to advance STOMP as a commercial “reservoir simulator” (similar to what oil companies use) for use in the methane hydrate industry. As a first step, White is modeling a recent DOE experiment that used the guest-molecule exchange method on the North Slope of Alaska. White will compare STOMP’s predictions to DOE’s actual results, and if they match well, it will establish confidence in the model’s ability and set the stage for supporting actual production operations.

While the focus of this research is on guest-molecule exchange, White points out that STOMP can be used to model any of the several methods being tried for methane hydrate production in the industry—making it a unique tool.

"I only know of a few other codes in the world that can do this," White says.

Next Steps

The model comparisons against laboratory- and field-scale experiments will continue for many years, yielding an analytical tool to aid scientists and engineers in the development of environmentally sound technologies for realizing the natural gas hydrate resource.

Acknowledgements

This research is being funded by the U.S. DOE Office of Fossil Energy Research and Development, Korea Institute of Geoscience and Mineral Resources, and ConocoPhillips.